Further characterization of [ 3H]ifenprodil binding in rat brain

Abstract

The present study was undertaken to characterize [ 3H]ifenprodil binding in rat brain. [ 3H]ifenprodil showed saturable, high-affinity binding at 4°C. Specific binding, defined with 10 μM ifenprodil as a competitor, was inhibited biphasically by the s receptor ligands, GBR 12909, 1,3-di- o-tolylguanidine (DTG), and (+)-3-(3-hydroxyphenyl)- N-propylpiperidine ((+)-3-PPP). At 4°C, 3 μM GBR 12909, which inhibited about 50% of specific binding of [ 3H]ifenprodil, was used to mask σ receptors. Under these conditions, specific binding of [ 3H]ifenprodil was inhibited potently by ifenprodil, SL 82.0715, poly( l-arginine), poly( l-lysine), neomycin, ruthenium red, spermine, arcaine and spermidine. In the presence of 3 μM GBR 12909, Zn 2+ and Mg 2+ partially inhibited specific binding of [ 3H]ifenprodil at 4°C. In contrast, in the absence of GBR 12909, at 37°C specific binding of [ 3H]ifenprodil was partially inhibited by Zn 2+, but not by Mg 2+. The anatomical distribution of [ 3H]ifenprodil binding at 4°C (GBR 12909 included) in rat brain closely paralleled that of [ 3H]MK-801 (dizocilpine) binding ( r = 0.971, P < 0.005). Without GBR 12909, specific [ 3H]ifenprodil binding at 37°C was inhibited potently by σ ligands. In the presence of 3 μM GBR 12909, [ 3H]ifenprodil binding at 4°C was highest in synaptosomal and myelin fractions; however, without GBR 12909, [ 3H]ifenprodil binding at 37°C was highest in microsomal and myelin fractions, consistent with the subcellular distribution of σ receptors. The results suggest that, in the presence of 3 μM GBR 12909, at 4°C, [ 3H]ifenprodil binds to sites that are sensitive to polyamines and related compounds; and that without GBR 12909, at 37°C, [ 3H]ifenprodil interacts with σ receptors in rat brain.